The present invention is directed to the production of semipermeable polybenzimidazole membranes.
In recent years there has been increasing interest expressed in the development of microporous membranes of a semipermeable nature which are useful in separating the components of a solution. For instance, semipermeable membranes have been investigated as a possible means to demineralize or purify otherwise unusable water and to thereby alleviate the increasing demands for potable water necessitated by rapid population and industrial growth in many parts of the world. Separation techniques which employ such membranes include electrodialysis, ultrafiltration, reverse osmosis, etc.
Electrodialysis separations employ an electrolytic cell having alternating anionic and cationic membranes that collect desalted and concentrated solutions in adjacent compartments. Such a technique can be useful to purify liquids by removing ionizable impurities, to concentrate solutions of electrolytes, or to separate electrolytes from non-electrolytes.
As opposed to the charge dependent types of separations, reverse osmosis utilizes pressure to transport materials (which may be either ionic or non-ionic) selectively through a membrane. Ultrafiltration, which is very similar, uses gravity or applied pressure to effect separation using membranes which act as submicronic sieves to retain large molecules and permit the passage of small, ionic, non-ionic forms.
The desalination of saline water (e.g., sea water) through the use of semipermeable membranes is commonly characterized by the use of pressure in excess of osmotic pressure and is therefore termed reverse osmosis. The natural tendency for a solution of a higher concentration separated from a solution of lower concentration by a semipermeable membrane is for the solvent on the side of lower concentration to migrate through the membrane to the solution of higher concentration thereby eventually equilibrating the concentrations of the two solutions. The degree of this natural tendency is measured in terms of osmotic pressure. The process may be reversed by applying a pressure to the side of higher concentration in excess of the osmotic pressure, thereby forcing the pure solvent of the solution of higher concentration through the semipermeable membrane to the side of lower concentration, thereby bringing about a separation. The natural tendency, which is believed to be the result of a difference in free energy resulting from the concentration gradient, is observed to operate at a high thermodynamic efficiency and at ambient temperature.
Semipermeable membranes proposed in the past have been formed from a variety of materials, and are characterized by the ability to permit one component (e.g., ions or molecules) of a solution to pass through the same to the substantial exclusion of other components (e.g., other ions or molecules). Examples of substances heretofore recognized to possess this property include cellophane (i.e., regenerated cellulose), cellulose esters (e.g., cellulose acetate, cellulose butyrate, etc.), animal or protein membranes, polyelectrolyte complexes, ethyl cellulose, crosslinked polyacrylates, etc.
The semipermeable membranes of the prior art are of limited applicability in many separatory processes because of inherent disadvantages relating to their chemical stability, strength, thermal stability, efficiency, length of life, and cost. Generally, the prior art membranes exhibit low thermal stability and therefore cannot be used successfully under conditions wherein the liquid undergoing treatment is at an elevated temperature. This may be a decided disadvantage in situations where the components to be separated only exist in solution at higher temperatures, or when it is economically advantageous to separate components of a solution at elevated temperatures rather than going through the expense of cooling it. Furthermore, some membranes exhibit a decided decrease in efficiency upon an increase in temperature or pressure thereby limiting their range of applicability. Solvent susceptibility may be another factor affecting the applicability of a particular porous membrane to a separation process. Additionally, semipermeable membranes may be inappropriate for a particular application due to low solute rejection values or low flux.
Other factors which render the semipermeable membranes of the prior art of limited usefulness in reverse osmosis separatory processes include their limited strength and chemical resistance and extremely short operating lives at high pressures and temperatures. Low strength properties have generally been manifest in the form of the inability of the prior art films to operate at pressures in excess of about 1,000 p.s.i. or to operate at lower pressures for extended periods of time, especially at temperatures in excess of about 50.degree. C. The use of such high pressures is quite desirable in order to increase the speed of reverse osmosis, particularly the speed at which desalinized water is formed. However, when such high pressures have been employed, operating efficiency (i.e., in reverse osmosis desalinization, the degree of salt removal from saline solutions) has significantly decreased. With the use of pulsating pumps in reverse osmosis separatory operations, the presence of rapid increases and decreases in the pressure applied to the reverse osmosis membrane has caused even greater problems when the use of prior art membranes has been attempted. Furthermore, in general commercial usage it is necessary that membranes be strong enough to withstand shipment, storage and general rough handling. Thus, the continued need to replace the prior art membranes due to mechanical failures has greatly limited their commercial usefulness.
The chemical resistance properties of the prior art separatory membranes have been their greatest shortcoming. Although the separation of solutions comprising only sodium chloride and water presents few chemical resistance problems to the prior art membranes, such pure solutions are rarely found. Many naturally occurring saline solutions contain materials which exhibit a degrading effect on previously known reverse osmosis membranes. For example, cellulose acetate and amide-linked polymers, such as those disclosed in U.S. Pat. No. 3,567,632, are subject to either base or acid catalyzed hydrolysis even in weakly basic or acidic solutions. Other compounds which may exhibit a degrading effect on the prior art membranes include formic acid, acetone and bisulfite ions.
Finally, many of the prior art semipermeable membranes are limited in their usefulness because of the low temperatures at which separatory operations must occur. Higher temperatures (e.g., those in excess of about 50.degree. C.) have resulted in reduced salt removal efficiency, particularly when extended operating times have been employed.
Representative cellulose acetate membranes which may be utilized in desalination process are disclosed in U.S. Pat. No. 3,133,132, issued to Loeb et al on May 12, 1964. The Loeb et al patent also discloses a process for preparing semipermeable membranes involving the casting of a cellulose acetate solution containing a pore-producing agent, (i.e., an agent which produces a structure which allows an appreciable rate of passage of fresh water under suitable conditions). It has been found, however, that cellulose acetate membranes described therein must be utilized under relatively mild conditions and may not satisfactorily be utilized at elevated temperatures (i.e , in excess of 70.degree. C. to 80.degree. C.). Upon continuous exposure to salt water such cellulose acetate membranes tend to undergo hydrolysis and become less effective for their intended purpose. Also, such membranes may be damaged by contact with a variety of solvents (e.g., phenol, acetone, methyl ethyl ketone, sodium hydroxide solutions, mineral acid solutions), or by bacteriological attack. Amide-containing membranes which may be utilized in desalination processes are disclosed in U.S. Pat. No. 3,567,632, issued to Richter et al on May 2, 1971. This patent discloses reverse osmosis desalination membranes prepared from nitrogen-linked aromatic polymers. These membranes, however, still exhibit many of the disadvantages previously noted for prior art semipermeable membranes and are therefore of limited usefulness. Although the Richter et al amide-linked polymer membranes may be operated at somewhat higher temperatures and possess greater strength than, for example, cellulose acetate membranes, the relative increases are still less than are commercially desirable. Generally, the useful life of such membranes at pressures in excess of about 300 to 400 p.s.i. is limited to about one to three months. After this period of operation both salt rejection percentage (in aqueous saline solutions) and desalinized water preparation speed has significantly decreased. Finally, these membranes are of limited chemical resistance, especially in view of their susceptability to aqueous hydrolysis in the presence of bases or acids.
The applicability of a particular membrane to the separation of components from solutions appears to depend on both the physical nature of the semipermeable structure and the particular chemical structure of the membrane. It should be noted here that, in accordance with common usage, the terms microporous and semi-permeable or permeable will be used interchangeably to denote the character or quality of the membrane which is necessary to render the membrane suitable for the use herein intended. More specifically, the membranes described herein are characterized by the fact that they allow one or more components of a solution to pass through them while they prevent the passage of one or more other components. Furthermore, the term membrane will be used to describe membranes whether prepared as a flat film, hollow fiber, or other form.
In an attempt to overcome certain of the above-noted disadvantages of prior art membranes, membranes comprised of polybenzimidazole polymers have been provided. See, for example, commonly-assigned U.S. Pat. Nos. 3,699,038; 3,720,607; 3,737,042; 3,841,492; 3,851,025; and 4,020,142, each herein incorporated by reference. While such membranes possess satisfactory properties, it has been proposed to subject such membranes to an annealing step to further enhance the properties thereof. See, in this regard, above-identified U.S. Pat. Nos. 3,699,038; 3,737,042; 3,841,492; and 3,851,025. Those patents disclose the use of annealing temperatures ranging up to about 300.degree. C. However, the use of annealing temperatures which exceed about 125.degree. C. to 160.degree. C. can result in disadvantageous mechanical properties in the resulting membrane such as severe dimensional change, membrane brittleness and cracking, etc. However, it has generally been found that annealing temperatures above 125.degree. C. to 160.degree. C. are necessary to achieve the desired enhancement of separation properties (e.g., salt separation) in the membranes.
The use of such annealing temperatures in excess of 100.degree. C. has rendered desirable the removal of water present in the membrane prior to annealing to avoid possibly disadvantageous vaporization of the water during annealing. The water has conventionally been removed by pretreatment of the membrane with ethylene glycol in separate baths of increasing concentration (e.g., 25 to 100 percent by weight of ethylene glycol) such that prior to annealing the water in the membrane is replaced by ethylene glycol which boils at a temperature in excess of the annealing temperature. Subsequent to annealing, the membrane is then passed through baths of decreasing ethylene glycol concentration to replace the ethylene glycol with water. Such pretreatment is, however, expensive and time-consuming.
It is thus an object of the invention to provide an improved process for the production of semipermeable polybenzimidazole membranes.
It is also an object of the invention to provide a process for the production of polybenzimidazole membranes which exhibit improved mechanical properties.
It is further an object of the invention to provide improved semipermeable polybenzimidazole membranes which may be utilized to separate components of a solution.
It is still further an object of the invention to provide improved semipermeable membranes possessing chemical and thermal stability.
It is further an object of the present invention to provide a process for the preparation of semipermeable membranes using a low temperature annealing step whereby the membrane which is produced exhibits desirable salt rejection characteristics.
These and other objects as well as the scope, nature and utilization of this invention will be apparent from the following detailed description.